A. Field of the Invention
The present invention relates to a semiconductor device and in particular to a semiconductor device composed of a plurality of semiconductor modules comprising power semiconductor chips, using a technique for connecting terminals of semiconductor modules.
B. Description of the Related Art
A power semiconductor module contains a power semiconductor chip(s), for example an insulated gate bipolar transistor(s) (IGBT(s)), mounted on an insulated circuit board and housed in a case. One type of dedicated package is prepared for power semiconductor module of a specified current carrying capacity rating. A large capacity power semiconductor module can hardly be obtained by producing a dedicated chip for a given current carrying capacity. Thus, usually employed packages have a structure which arranges in parallel a plurality of power semiconductor chips with a specified current carrying capacity.
In a large capacity semiconductor module of silicon IGBT in a class of several hundred amps to 1,000 amps, a package includes several to several tens of semiconductor chips. In recent years, semiconductor modules using SiC semiconductor chips have been developed for composing an inverter. For the semiconductor modules using SiC semiconductor chips, which have generally small current carrying capacity, even a package with a current rating of from several tens of amps to 100 amps have a multiple of SiC semiconductor chips connected in parallel. Thus, producing a large capacity semiconductor module at a class of several hundred amps to 1,000 amps needs, in principle, several tens to several hundreds of SiC semiconductor chips arranged in parallel. This prevents the SiC semiconductor chips from commercial application.
In a semiconductor module completely dedicated to a certain current carrying rating, if only one chip or part becomes defective in the assembling process, the entire semiconductor module becomes defective and has to be discarded. In operation of the semiconductor module, if one chip or part breaks down, the entire semiconductor module must be replaced. Therefore, a dedicated package of large capacity, in particular, becomes costly for the manufacturers of the semiconductor module and risky for the users.
Some dedicated packages of large capacity semiconductor module comprise a multiple of medium capacity common parts in a unit with an insulated board, the common parts being arranged in parallel in the number necessary for providing a specified current capacity. The terminals and casing, on the other hand, are prepared in the arrangement and configuration just corresponding to the large current carrying capacity. A package composed of a multiple of medium capacity parts in a unit with an insulated board can avoid such discard of the entire semiconductor module that is caused by defective assembly at least up to the level of the insulated board unit. Nevertheless, in the later assembly steps and operation, breakdown of only one semiconductor chip or a part can result in discard and replacement of the entire semiconductor module.
Another approach achieves large capacity by arranging a number of discrete small capacity, small sized packages by mounting small amount of semiconductor chips and connecting with special attaching jig. Although the problem mentioned above does not take place in arrangement of many discrete small capacity, small-sized packages, new problems may arise in that the user attaches many parts or the user is forced to attach in a substantially different arrangement. In addition, the small size of package may fail to satisfy the insulation distance requirement.
An example of a dedicated package has the following structure. A plurality of semiconductor chips are connected and mounted on an insulated circuit board with solder. The semiconductor chips are wired to the insulated circuit board with bonding wires. A metallic base for heat dissipation is soldered to the surface of the insulated circuit board opposite to the surface on which the semiconductor chips are mounted. On the insulated circuit board, a bus bar including external terminals is connected and mounted with screw fitting or soldering. The metallic base for heat dissipation is fixed to a casing in which the semiconductor chip is sealed with gel filled in the casing. The external terminals of the bus bar project out of the casing. A dedicated package for use at temperatures above 200° C. preferably has a structure in which the semiconductor chip is sealed, in place of the gel, with resin molding that simultaneously works as a case. However, molding of a large sized semiconductor module can cause a problem of lowered yield rate due to occurrence of cracks and warp, and increased material costs.
It is difficult to connect a bus bar and another part by screwing if sufficient space cannot be secured around the bus bar. It is also difficult to connect them securely by soldering because the joining part cannot be made at a high temperature necessary for the soldering due to the high thermal conductivity of the material, for example copper, of the bus bar. If a large amount of heat is given to the joining part in order to carry out soldering, the heat through the joining part with the bus bar adversely affects the semiconductor module. Moreover, the soldering process, usually employing wire solder, can leave a residue of solder flux at the joint after the soldering process. The residue of flux is hardly cleaned altogether with the semiconductor module.
Patent Document 1 discloses a semiconductor device with various current carrying capacities of semiconductor modules obtained by solder joining externally led out terminals of semiconductor units and a wiring substrate having a wiring pattern formed thereon. The device of Patent Document 1, however, has the above-mentioned problem of flux residue because the joint is formed by soldering the externally led out terminals and the wiring substrate.
Patent Document 2 discloses a method of joining the base part of a terminal of an electronic component and a fine wire of lead wire by welding with electron beam or laser beam. The document, however, discloses only a method of joining a terminal and a lead wire, and does not mention about joining a bus bar of a semiconductor module and another part, for example, externally connecting terminals by means of the electron beam or laser beam welding.
Patent Document 3 discloses a method of joining a plate conductor and a wire by means of arc welding, especially tungsten inert gas (TIG) welding. However, when a bus bar of a semiconductor module and another part, for example, an externally connecting terminal are joined by means of the arc welding disclosed in Patent Document 3, this can adversely affect the semiconductor module through the terminal.
Patent Document 4 discloses a method of joining a printed circuit board and a junction terminal by putting a tab of the terminal through a through-hole of the board and then irradiating a pulse laser at the joint. The method, however, can only join a tab thinner than the body of the junction terminal, and does not join the junction terminal itself.
Patent Document 1—Japanese Unexamined Patent Application Publication No. 2011-142124
Patent Document 2—Japanese Unexamined Patent Application Publication No. S63-130291
Patent Document 3—Japanese Unexamined Patent Application Publication No. 2001-219270
Patent Document 4—Japanese Unexamined Patent Application Publication No. 2002-025639
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.